Machines performing expansion or compression of gases or vapours may be classified into turbo-machines and positive displacement machines. Advantages associated with turbo-machines include great specific power, no need of power transmitting mechanisms, small wear and suitability for use of relatively cheap fuels in connection with a heat power process. Disadvantages include losses of 10-20% in consequence of kinetic way of action, poor suitability for light gases as helium or hydrogen and a necessity to have the first part of a turbine at the maximum temperature of gas or steam in connection with a heat power process.
Typical drawbacks associated with known positive displacement machines include the need of power transmitting mechanisms, low specific power and disadvantages associated with sealing lubrication, namely power losses, temperature limitations as well as rapid wear. On the other hand, types not using sealing lubrication are generally characterized by a great gas leak due to structures where sizes of leaking clearances depend on mutual synchronization between moving members, said synchronization being imperfect e.g. because of free play in gears. The necessity of transmitting power through those mechanisms makes it further more difficult to avoid said play. The lack of possibilities to reduce the speed of gas leak to a value lower than the velocity of sound is another main reason for great losses. Moreover, structures of some machines are suitable for low pressure levels only.
Also, the state of the art does not offer a profitable method for storing electric energy on a large scale, particularly for longer periods. Known possibilities include storing as potential energy by pumping water into a reservoir situating on a higher level, a production of hydrogen, compressed air storage in caves, super-conducting magnetic energy storage, flywheel storage and electrochemical accumulators. The drawback of the first mentioned alternative is that a size of a reservoir is so enormous that even diurnal storing would encounter difficulties, not to mention an annual storing. The situtation is quite similar in the case of compressed air storage, which might be fitted for short term storage in some special sites. The only known method suitable for a long term storage would be a production of hydrogen, but its drawback is a poor efficiency due to losses in production of hydrogen, in storing as well as in a heat power process after storing. Also a danger of an explosion and diffuculties in storing in consequence of a low boiling point of hydrogen may be considered serious disadvantages. The other alternatives mentioned are far from profitable, particularly for long term storage.